// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_SCOPED_GENERIC_H_
#define BASE_SCOPED_GENERIC_H_

#include <stdlib.h>

#include <algorithm>

#include "base/compiler_specific.h"
#include "base/macros.h"

namespace base {

// This class acts like unique_ptr with a custom deleter (although is slightly
// less fancy in some of the more escoteric respects) except that it keeps a
// copy of the object rather than a pointer, and we require that the contained
// object has some kind of "invalid" value.
//
// Defining a scoper based on this class allows you to get a scoper for
// non-pointer types without having to write custom code for set, reset, and
// move, etc. and get almost identical semantics that people are used to from
// unique_ptr.
//
// It is intended that you will typedef this class with an appropriate deleter
// to implement clean up tasks for objects that act like pointers from a
// resource management standpoint but aren't, such as file descriptors and
// various types of operating system handles. Using unique_ptr for these
// things requires that you keep a pointer to the handle valid for the lifetime
// of the scoper (which is easy to mess up).
//
// For an object to be able to be put into a ScopedGeneric, it must support
// standard copyable semantics and have a specific "invalid" value. The traits
// must define a free function and also the invalid value to assign for
// default-constructed and released objects.
//
//   struct FooScopedTraits {
//     // It's assumed that this is a fast inline function with little-to-no
//     // penalty for duplicate calls. This must be a static function even
//     // for stateful traits.
//     static int InvalidValue() {
//       return 0;
//     }
//
//     // This free function will not be called if f == InvalidValue()!
//     static void Free(int f) {
//       ::FreeFoo(f);
//     }
//   };
//
//   typedef ScopedGeneric<int, FooScopedTraits> ScopedFoo;
template <typename T, typename Traits>
class ScopedGeneric {
private:
    // This must be first since it's used inline below.
    //
    // Use the empty base class optimization to allow us to have a D
    // member, while avoiding any space overhead for it when D is an
    // empty class.  See e.g. http://www.cantrip.org/emptyopt.html for a good
    // discussion of this technique.
    struct Data : public Traits {
        explicit Data(const T& in)
            : generic(in)
        {
        }
        Data(const T& in, const Traits& other)
            : Traits(other)
            , generic(in)
        {
        }
        T generic;
    };

public:
    typedef T element_type;
    typedef Traits traits_type;

    ScopedGeneric()
        : data_(traits_type::InvalidValue())
    {
    }

    // Constructor. Takes responsibility for freeing the resource associated with
    // the object T.
    explicit ScopedGeneric(const element_type& value)
        : data_(value)
    {
    }

    // Constructor. Allows initialization of a stateful traits object.
    ScopedGeneric(const element_type& value, const traits_type& traits)
        : data_(value, traits)
    {
    }

    // Move constructor. Allows initialization from a ScopedGeneric rvalue.
    ScopedGeneric(ScopedGeneric<T, Traits>&& rvalue)
        : data_(rvalue.release(), rvalue.get_traits())
    {
    }

    ~ScopedGeneric()
    {
        FreeIfNecessary();
    }

    // operator=. Allows assignment from a ScopedGeneric rvalue.
    ScopedGeneric& operator=(ScopedGeneric<T, Traits>&& rvalue)
    {
        reset(rvalue.release());
        return *this;
    }

    // Frees the currently owned object, if any. Then takes ownership of a new
    // object, if given. Self-resets are not allowd as on unique_ptr. See
    // http://crbug.com/162971
    void reset(const element_type& value = traits_type::InvalidValue())
    {
        if (data_.generic != traits_type::InvalidValue() && data_.generic == value)
            abort();
        FreeIfNecessary();
        data_.generic = value;
    }

    void swap(ScopedGeneric& other)
    {
        // Standard swap idiom: 'using std::swap' ensures that std::swap is
        // present in the overload set, but we call swap unqualified so that
        // any more-specific overloads can be used, if available.
        using std::swap;
        swap(static_cast<Traits&>(data_), static_cast<Traits&>(other.data_));
        swap(data_.generic, other.data_.generic);
    }

    // Release the object. The return value is the current object held by this
    // object. After this operation, this object will hold a null value, and
    // will not own the object any more.
    element_type release() WARN_UNUSED_RESULT
    {
        element_type old_generic = data_.generic;
        data_.generic = traits_type::InvalidValue();
        return old_generic;
    }

    const element_type& get() const { return data_.generic; }

    // Returns true if this object doesn't hold the special null value for the
    // associated data type.
    bool is_valid() const { return data_.generic != traits_type::InvalidValue(); }

    bool operator==(const element_type& value) const
    {
        return data_.generic == value;
    }
    bool operator!=(const element_type& value) const
    {
        return data_.generic != value;
    }

    Traits& get_traits() { return data_; }
    const Traits& get_traits() const { return data_; }

private:
    void FreeIfNecessary()
    {
        if (data_.generic != traits_type::InvalidValue()) {
            data_.Free(data_.generic);
            data_.generic = traits_type::InvalidValue();
        }
    }

    // Forbid comparison. If U != T, it totally doesn't make sense, and if U ==
    // T, it still doesn't make sense because you should never have the same
    // object owned by two different ScopedGenerics.
    template <typename T2, typename Traits2>
    bool operator==(
        const ScopedGeneric<T2, Traits2>& p2) const;
    template <typename T2, typename Traits2>
    bool operator!=(
        const ScopedGeneric<T2, Traits2>& p2) const;

    Data data_;

    DISALLOW_COPY_AND_ASSIGN(ScopedGeneric);
};

template <class T, class Traits>
void swap(const ScopedGeneric<T, Traits>& a,
    const ScopedGeneric<T, Traits>& b)
{
    a.swap(b);
}

template <class T, class Traits>
bool operator==(const T& value, const ScopedGeneric<T, Traits>& scoped)
{
    return value == scoped.get();
}

template <class T, class Traits>
bool operator!=(const T& value, const ScopedGeneric<T, Traits>& scoped)
{
    return value != scoped.get();
}

} // namespace base

#endif // BASE_SCOPED_GENERIC_H_
